This invention relates to error correction coding systems and, more particularly, to a rate one-half convolutional coding system capable of correcting one random error for each four successive bits in a bit stream.
Error correcting codes are well known in the art, especially in the digital computer and digital data communication fields. Errors may occur whenever a stream of digital information bits, i.e., a sequence of logic "0" or "1" levels, is transmitted over a channel subject to noise interference. It has been found that induced channel error can be detected and corrected if parity bits are transmitted along with the information bits. The parity bits are related to the information bits in a predetermined manner such that upon reception errors in transmission may be found. Parity bits may be sent in blocks, or they can be convolved with the information bits. The rate of an encoding system is defined as the ratio of the number of information bits to the total number of bits in a coded transmission. The present invention is addressed to convolutional codes having a rate of one-half, i.e., one parity bit for every information bit.
Many encoding schemes have been devised but, until my recent discovery, which is the subject of U.S. patent application Ser. No. 438,138, filed Jan. 30, 1974, now U.S. Pat. No. 3,882,457, and assigned to the same assignee as the instant invention, no code realized the theoretical maximum of correcting one error in each four successive transmitted bits. A drawback with the aforementioned encoding system is that, in some applications, it requires a minimum six bit delay for each message transmitted. Not only is the delay detrimental to swift communication, but it also requires more hardware.